1. Field of the Invention
The invention disclosed in this specification relates to a thin film transistor and a fabrication method thereof and more particularly to a circuit and a device constructed by using a thin film transistor.
2. Description of Related Art
Hitherto, there has been known a thin film transistor (hereinafter referred to as a TFT or the like) constructed by forming a thin film semiconductor or a silicon semiconductor film in particular on a substrate.
While such a TFT is used in various integrated circuits, it is often used in an active matrix type liquid crystal display in particular. The active matrix type liquid crystal display has a structure in which the TFT is disposed as a switching device in each pixel electrode arranged in a matrix. There has been also known a liquid crystal display in which not only the matrix circuit but also a peripheral driving circuit thereof is constructed by TFTs (which is called a peripheral driving circuit integrated display).
The TFT is also used in various integrated circuits and multi-layered integrated circuit (three-dimensional IC).
It is convenient to use an amorphous silicon film which is formed by means of chemical vapor deposition such as plasma CVD as a silicon film used in the TFT. It may be said that this technology has been almost established.
However, the TFT using the amorphous silicon film has an electrical characteristic which is far lower than that using a single crystal semiconductor used in general semiconductor integrated circuits. Therefore. it is the present situation that it is used only in the limited uses such as a switching device of the active matrix circuit.
As a technological trend of the future, it is required to realize an arrangement in which the active matrix circuit, the peripheral driving circuit, an image processing circuit, an oscillation circuit and the like are integrated on one and the same substrate.
A crystal silicon film may be used, instead of the amorphous silicon film, in order to improve the characteristic of the TFT using the amorphous silicon film. A silicon film having a crystallinity beside single crystal silicon is called poly-crystal silicon, poly-silicon, microcrystal silicon and the like.
Such a silicon film having the crystallinity may be obtained by forming the amorphous silicon film at first and then by crystallizing it by heating (annealing). This method is called solid phase growth because the amorphous state changes to the crystal state while keeping the solid state.
However, there has been a problem that the solid phase growth of silicon requires more than 600xc2x0 C. of heating temperature and more than 20 hours of heating time and it is difficult to use a low cost glass substrate as a substrate.
For example, the Corning 7059(copyright) glass used for the active matrix type liquid crystal display has a glass strain point of 593xc2x0 C. and there is a problem in performing the annealing at 600xc2x0 C. or more when the increased area of the substrate is taken into consideration.
There has been also another problem in terms of productivity that it takes more than 20 hours for the heat treatment for crystallization.
In order to solve such problems, the inventors had developed a technology which allows the crystallization to be achieved at 550xc2x0 C. in about 4 hours of treatment time by depositing a trace amount of a certain kind of metal element such as nickel and palladium on the surface of the amorphous silicon film and then by heating it (Japanese Patent Laid-Open No. 6-244103, the disclosure thereof being incorporated herein by reference).
It is possible to obtain a silicon film having a better crystallinity when it is annealed at 600xc2x0 C. for 4 hours.
This technology allows a crystal silicon film having a large area to be obtained on a low cost glass substrate with a high productivity.
As methods for introducing such a trace amount of metal element (metal element which promotes the crystallization), there are methods of depositing a coating film of the metal element or its compound by means of sputtering as disclosed in Japanese Patent Laid-Open No. 6-244104, of forming a coating film of the metal element or its compound by means of spin coating or the like as disclosed in Japanese Patent Laid-Open No. 7-130652 and of forming a coating film by decomposing gas containing the metal element by means of thermal decomposition, plasma decomposition or the like as disclosed in Japanese Patent Laid-Open No. 7-335548. The disclosure of these Laid-Opens are incorporated herein by reference.
There is also a method of selectively introducing the metal element to a specific part and of then widening the growth of crystal from the part where the metal element has been introduced to the peripheral part (lateral growth method). The crystal silicon obtained by such a method has an oriented crystal structure and shows very excellent characteristics in response to the orientation.
The methods for fabricating the crystal silicon film by using a certain kind of metal element, e.g. nickel, is very excellent as described above. However, it has been found that there are problems when a TFT is fabricated by using such a crystal silicon film that its device characteristic varies and its reliability is low.
Accordingly, it is an object of the invention disclosed in the present specification to provide a technology which allows a TFT whose device characteristic varies less to be obtained in fabricating the TFT by using the crystal silicon film obtained by using metal element.
According to one aspect of the invention disclosed in the present specification, a method for fabricating a semiconductor device comprises, as its one exemplary fabrication steps are shown in FIGS. 1A through 2I, steps of forming a crystal silicon film 107 on an insulated surface by using metal element which promotes crystallization of silicon (FIGS. 1A and 1B); forming a mask 109 on the crystal silicon film (FIG. 1C); gettering the metal element to specific regions 111 and 112 of the crystal silicon film by using the mask 109 (FIG. 2E); and forming an active layer 116 of a device by using the mask 109 (which turns out to be a part 115 as its side is etched) (FIG. 2H).
According to another arrangement of the invention, a method for fabricating a semiconductor device comprises steps of forming a crystal silicon film on an insulated surface by using metal element which promotes crystallization of silicon; forming a mask on the crystal silicon film; selectively doping element selected among nitrogen, phosphorus, arsenic antimony and bismuth to the crystal silicon film by using the mask; performing a heat treatment to getter the metal element to regions which have been doped; and removing the doped regions by using the mask. In the arrangement described above, what is most effective as the dopant is phosphorus.
According to a still other arrangement of the invention, a method for fabricating a semiconductor device comprises steps of forming a crystal silicon film on an insulated surface by using metal element which promotes crystallization of silicon; forming a mask on the crystal silicon film; selectively doping element selected among nitrogen, phosphorus, arsenic, antimony and bismuth to the crystal silicon film by using the mask; performing a heat treatment to getter the metal element to regions which have been doped; and forming an active layer of a device by using the regions from which the metal element has been gettered by utilizing the mask.
According to a still other arrangement of the invention, a method for fabricating a semiconductor device comprises, as its concrete fabrication steps are shown in FIGS. 1A through 2I, steps of forming a crystal silicon film 107 on an insulated surface by using metal element which promotes crystallization of silicon (FIGS. 1A and 1B); forming a mask 109 on the crystal silicon film 107 (FIG. 1C); selectively doping element selected among nitrogen, phosphorus, arsenic, antimony and bismuth (phosphorus in this case) to the crystal silicon film by using the mask 109 (FIG. 1D); performing a heat treatment to getter the metal element to regions 111 and 112 which have been doped (FIG. 2E); and etching regions of the gettered regions adjacent to the doped regions in a manner of self-alignment by utilizing the mask 113 (FIG. 2H). It is possible to introduce other elements instead of phosphorus as far as gettering effects can be expected.
The above-mentioned steps are characterized in that phosphorus is doped by using the mask 109 and that a pattern 116 is obtained by using the pattern 115 which has been obtained by side-etching the mask 109.
It allows the regions of the mask 113 adjacent to regions 111 and 112 to be removed and to suppress nickel element from influencing on the region 116.
In the invention disclosed in the present specification, it is most preferable to use Ni (nickel) as the metal element which promotes the crystallization of silicon.
Further, one or a plurality of types of metal elements selected among Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au may be used as the metal element which promotes the crystallization of silicon.
Still more, a compound film represented as SixGe1xe2x88x92x(0 less than x less than 1) may be used instead of the crystal silicon film. In this case, the amorphous silicon film, i.e. the starting film, may be made of the compound film represented as SixGe1xe2x88x92x(0 less than x less than 1).
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings.